This invention relates to a process for the preparation of halophthalic anhydrides by dehydrogenation of halogen substituted saturated or partially saturated phthalo compounds, such as halogen, substituted tetrahydro or hexahydro phthalic anhydrides. Halophthalic anhydrides are useful chemical intermediates for the synthesis of various commercial products, including polymers, dyes and plasticizers.
The increasing importance of high performance polyimides has led to an increased interest in halophthalic anhydrides. The latter are particularly useful as intermediates for the preparation of dianhydride monomers, such as oxydiphthalic anhydride which may be co-polymerized with a suitable diamine to form a condensation polyimide. The preparation of dianhydride monomers for the high performance polymer industry requires halophthalic anhydrides of very high purity, since the presence of even what normally would be considered as minor amounts of impurities would degrade the polymer product and perhaps render the product unsuitable for certain uses.
Halophthalic anhydrides may be prepared by the reaction of bromine with halo-substituted saturated or partially saturated phthalic anhydrides, such as halotetrahydrophthalic anhydride or gem-dihalohexahydrophthalic anhydride, at temperatures in excess of 200.degree. Celsius. However, this approach has been found to result in relatively low yields and is in general, uneconomical.
Various other methods for the preparation of phthalic anhydrides by the dehydrogenation of saturated or partially saturated cyclic anhydrides are known in the chemical literature.
Bergmann J. Amer. Chem. Soc. 64, 176 (1942) discloses the aromatization of tetrahydrophthalic anhydride products of Diels-Alder reactions. The author discloses that dehydrogenation occurred when the tetrahydrophthalic anhydride product is boiled in nitrobenzene. However, it is further disclosed that dehydrogenation does not occur when p-bromonitrobenzene, p-chloronitrobenzene, or m-dinitrobenzene in xylene is employed. Moreover, it has been found that when the dihalohexahydrophthalic anhydrides are dehydrogenated in nitrobenzene, a portion of the nitrobenzene is reduced to aniline. The aniline reacts with the anhydride group of either the starting material or product to form imides and thus lower the yield of desired product.
U.S. Pat. No. 4,560,772 to Telschow discloses the reaction of 4-methyltetrahydrophthalic anhydride with excess sulfur and a catalytic amount of zinc oxide and 2-mercaptobenzothiazole to produce 4-methylphthalic anhydride and hydrogen sulfide.
U.S. Pat. No. 4,560,773 and 4,559,405 to Telschow disclose the preparation of substituted phthalic anhydrides by reaction of bromine with an alkyl or aryl-substituted tetrahydrophthalic anhydride, especially 4-methyltetrahydrophthalic anhydride, in the presence of an acid acceptor, such as pyridine or dimethylformamide. In the working examples, U.S. Pat. No. 4,560,773 discloses yields of 62-82% and purity of only 90-95% even after vacuum distillation. According to the patentee, the yield and purity of the desired end product would be even lower if the reaction were carried out in the absence of an acid acceptor.
U.S. Pat. No. 4,517,372 to Tang, disclose a process for the preparation of 4-fluorophthalic anhydride by dehydrogenation of gem-, difluoro- or gem-chloro-fluoro- hexahydrophthalic anhydrides in the presence of a dehydrogenation catalyst, such as palladium.
U.S. Pat. No. 4,709,056 to Cotter, Lin, and Pawlak discloses the preparation of 4,4-difluorohexahydrophthalic anhydride and 4-chloro-4-fluorohexahydrophthalic anhydrides by reaction of hydrogen fluorides with 4-chlorotetrahydrophthalic anhydride.
Skvarchenko et al., Obshchei Khimii, Vol. 30, No. 11. pp. 3535-3541 disclose the aromatization of chloro-substituted tetrahydrophthalic anhydride by heating with phosphorus pentoxide. In the aromatization process described, however, decarboxylation also occurs with the formation of the corresponding chloro-substituted benzene compound. The preparation of various other tetrahydrophthalic acids, and anhydrides and various methods for dehydrogenation and aromatization thereof are reviewed by Skvarchenko in Russian Chemical Review. No. 1963, pp. 571-589.
Co-pending application Ser. No. 07/393,449, which is a C-I-P or Ser. No. 160,033 and Ser. No. 160,034, is directed to the preparation of halophthalic anhydrides by the reaction of chlorine with halotetrahydrophthalic anhydride or gem-dihalohexahydrophthalic anhydride at temperatures of 200.degree. Celsius and higher.
Although the chemical literature discloses a variety of methods for the preparation of substituted phthalic anhydrides, it will be appreciated that a need continues to exist for a more economical and efficient dehydrogenation process, suitable for the preparation of high purity halophthalic anhydrides.